Not all marine deep cycles are dual purpose. They are engineered for totaly different purposes.
One is a "true" deep cycle engineered for trolling motor use and is used by most tournament or avid fishermen.
The marine deep cycle/starting battery is used not only as a starting battery, but power the electronics, ie. fish locators, lights, depth sounders, live well pumps etc.
The internal construction of a marine battery is quite different than a standard deep cycle and is not needed in an RV, plus they are more expensive.